As semiconductor devices continually decrease in size, the use of flip chip assemblies generally gains in popularity. Flip chip assemblies are semiconductor chips, or integrated circuits, that have external electrical pads on a surface of the chip to which solder pillars or bumps are connected. The pillars or bumps are then connected to a substrate or carrier using a soldering process. The pillars or bumps create a physical and electrical connection between the substrate and the chip. Further, generally an underfill is used to fill the space between substrate and the chip and around the pillars or bumps to give the flip chip assembly more mechanical strength.
The pillars or bumps typically comprise copper, although many forms of solder may be used. Further, the pillars or bumps are generally coated or immersed in tin to protect the bumps from external factors, such as corrosion, as well as to aid in the soldering process. However, by immersing the pillars or bumps in tin, voids may randomly occur at the interface between the copper in the pillar or bump and the tin. The voids may be generally around 200 nanometers wide. The voids may lead to weakened mechanical strength of the connection, may cause delamination of a semiconductor chip, and may lead to cracks in the pillars or bumps that can cause open electrical circuits.
The cause of the voids is not readily apparent. One theorized cause is that the presence of copper oxide or irregularities at the copper-tin interface prevents tin from reacting or bonding to the copper and forms irregular voids. Also, by depositing tin at a high temperature onto the copper, the reaction between the copper and the tin may cause lateral shear stresses that can cause the voids.